Two Pathways for Electrocatalytic Oxidation of Hydrogen by a Nickel Bis(diphosphine) Complex with Pendant Amines in the Second Coordination Sphere

A nickel bis­(diphosphine) complex
containing pendant amines in
the second coordination sphere, [Ni­(PCy2Nt‑Bu2)2]­(BF4)2 (PCy2Nt‑Bu2 = 1,5-di­(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane),
is an electrocatalyst for hydrogen oxidation. The addition of hydrogen
to the NiII complex gives three isomers of the doubly protonated
Ni0 complex [Ni­(PCy2Nt‑Bu2H)2]­(BF4)2. Using the pKa values and
NiII/I and NiI/0 redox potentials in a thermochemical
cycle, the free energy of hydrogen addition to [Ni­(PCy2Nt‑Bu2)2]2+ was determined to be −7.9 kcal mol–1. The catalytic rate observed in dry acetonitrile
for the oxidation of H2 depends on base size, with larger
bases (NEt3, t-BuNH2) resulting
in much slower catalysis than n-BuNH2.
The addition of water accelerates the rate of catalysis by facilitating
deprotonation of the hydrogen addition product before oxidation, especially
for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior
to oxidation, leads to an overpotential that is 0.38 V lower compared
to the pathway where oxidation precedes proton movement. Under the
optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency
of 58 s–1 is observed at 23 °C.